172 



ELECTRICAL MEASUREMENTS 



per cent, allbws the percentage difference of the two coils to be 

 calculated nearly enough for practical purposes. The 1-ohm coil 

 is thus compared with the standard ohm, then the 2-ohm coil 

 with the sum of the 1-ohm and the standard, then the second 

 2-ohm coil with the first and so on. 



The 10-ohm coil may be compared with a 10-ohm standard and 

 the values of the 20-ohm, 50-ohm, 100-ohm and other coils 

 determined by comparison with those previously calibrated, or 

 the whole series may be built up from the standard ohm. 



After one box has been calibrated, others may readily be com- 

 pared with it by this method. 



FIG. 97. Arrangement for compensating large thermo-electromotive 



forces. 



Compensation for Large Thermo-e.m.f. Occasionally in 

 special bridge arrangements the unavoidable inequalities of 

 temperature in the apparatus may cause thermo-e.m.fs. of 

 sufficient magnitude to drive the galvanometer spot off the 

 scale. In many cases such e.m.fs. may be compensated, if 

 they are reasonably constant and the galvanometer can be used 

 on closed circuit. 



Referring to Fig. 97, R is a high resistance of several thousand 

 ohms, S.W. a slide wire; by adjusting the slider, the galvanometer 

 may be brought to zero; then the bridge is balanced as usual. 



Slide-wire or Divided-meter Bridge. In this simple form 

 of Wheatstone bridge, shown in Fig. 98, two of the arms are re- 

 placed by the two sections of a uniform wire. 



In the diagram the heavy lines represent copper strips of low 

 resistance with gaps at a, b, c, d. On each side of each gap is a 



